By definition, unit magnetization or more specifically, unit magnetic moment in the centimeter-gram-second (CGS) system is possessed by a magnet formed by magnetic poles of opposite sign and of unit strength, one centimeter apart. The magnetic moment may be expressed for example as units per gram or per cubic centimeter. Techniques for measuring the magnetic moment of a material generally fall into several broad classes, one of which is now known as a vibrating sample magnetometer. An exemplary vibrating sample magnetometer for measuring magnetic moment which measures the change in flux when a sample is vibrated in a substantially uniform magnetic field is described in U.S. Pat. No. 2,946,948 of the present inventor. A detector such as an inductive coil pickup is positioned adjacent to the vibrating sample to detect the change of magnetic flux produced by the moving sample and a reference detector is disposed adjacent to a reference magnet to generate a signal with a known phase relationship to the motion of the sample. The output signal, indicating magnetic flux change which is proportional to the sample magnetic moment, and the reference signal are differentially combined to provide a null output or a difference signal.
Improvements of the vibrating sample magnetometer of the aforementioned U.S. Pat. No. 2,946,948 are described in U.S. Pat. No. 3,496,459, also of the present inventor. According to that patent, the output signals from the magnetometer sensor are analyzed at a frequency which is a harmonic of the vibration frequency of the sample. A further improvement in such a magnetometer includes circuitry to compensate for out-of-phase signals including eddy currents as is described in copending U.S. patent application Ser. No. 513,090, filed Oct. 8, 1974, also of the present inventor.
An alternating force magnetometer analogous to the vibrating sample magnetometer described above, but in which force is applied to a sample by subjecting the sample to a periodically varying magnetic field gradient, is described in a paper by R. Reeves, entitled, "An Alternating Force Magnetometer," page 547, Journal of Physics E: Scientific Instruments, 1972, Volume 5.
Magnetometers in which a sample is placed in the vicinity of a vibrating coil are also known. In such magnetometers, a background AC signal is generated by vibration of the sensing coil in the region of the sample in the presence of the applied magnetic field. The AC output of the vibrating coil, however, reflects nonuniformities of the applied field and nonuniform fields produced by materials surrounding the sample or other outside factors. This dependency which may vary with time, makes measurements of the sample moment by a vibrating coil magnetometer susceptible to these background errors.
A rotating sample magnetometer is also described in articles by P. J. Flanders, entitled "Utilization of a Rotating Sample Magnetometer," Review of Scientific Instruments, Volume 41, No. 5, pp. 697-710, May 1970, and in an article by Stephen J. Hudgens, entitled "Rotating Sample Magnetometer for Diamagnetic Measurements," page 579, Review of Scientific Instruments, Volume 44, No. 5, May 1973.